How To Create A Circular Camera "Safe Frame"
There are a couple of ways to do what you want.


Use an image.


Create an image that has the shape you want to use as "safety".



Then in the 3d vieport enable background images, select the image you created, set it to be used for camera view only. You can then choose to use it as background or foreground and adjust the opacity.

 


Use a "child of" constraint.


Select the circle object, add a "child of" constraint. Select the camera to be used  as target. The circle will move to where the camera is located and will be rotated to match the camera rotation.

Manually postition the object so that is visible within the camera and scale it to your needs.

Since the object is parented to the camera, moving the camera will move the object as well.